This is a TangoDown Grip, model BGV-MK46FDE.
The Vertical Foregrip that started it all. The BGV-MK46 has become the standard for US Military small arms" offering levels of utility previously unseen in this type of product. Rugged ergonomic design is complimented by premium heat and impact-modified injection molded polymers for long mission life. The BGV-MK46 is at home on MK18s" M4s & M16s as it is on M32 40mm grenade launchers & MK 48 MOD 1 belt-fed machine guns.
Features:
Flat dark earth finish
Fits Picatinny
Waterproof internal storage compartment w/o-ring screw cap